Traditionally cellular networks have been circuit switched networks, i.e. a certain part of the data transmission capacity both at the fixed network and in the radio access network is reserved for each call. This capacity is reserved during the whole call, even if no speech or other data is transmitted.
The popularity of applications that can be run conveniently over packet switched networks, applications such as electronic mail and browsing the World Wide Web, has caused need to enhance current cellular networks to support packet switched connections. For example, in Global System for Mobile communication (GSM) packet switched connections are provided by General Packet Radio Service (GPRS). Existing GSM networks can be updated to carry packet data by adding proper network elements. With GPRS it should be possible, for example, use the radio resources in the radio access network more economically than by transmitting packet data in a circuit switched connection, i.e. in a data call, as can be done in GSM.
Universal Mobile Communication System (UNITS) is one of the future cellular networks that offer larger data transmission capacity than current cellular networks. UMTS supports packet switched connections, and same GPRS equipment as in GSM networks may be used there. In fact, GSM radio access networks and UMTS radio access networks may be connected to a common GPRS core network.
FIG. 1 presents a schematic diagram of an exemplary GSM radio access network and GPRS core network. A mobile station (MS) 101 communicates with a base station (BTS) 102. One or more base stations are connected to a base station controller (BSC) 103 that is responsible, for example, for allocation of radio resources and for handling handovers, where a mobile station changes the base station it communicates with. The base stations and base station controllers form the GSM radio access network. In addition to these components, a GSM network comprises Mobile Service Switching centers (MSC), Home Location Register (BLR) and Visitor Location Register (VLR). ELR and VLR take part in, for example, subscriber and mobility management.
The GPRS core network comprises GPRS supporting nodes (GSN). Of these nodes, the one which is on the edge towards a public data network, for example the Internet, is called Gateway GPRS supporting node (GGSN). In FIG. 1, a GGSN 105 is presented. Data packets may run through many GSNs, which act as routers. A mobile station, which is the endpoint of the data connection, is reachable through one base station controller and the GSN connected to this base station controller is called Serving GPRS support node (SGSN). In FIG. 1, the mobile station 101 is reachable via the BSC 103 and the GSN connected to this BSC is SGSN 104.
User data is transferred transparently between the MS and the external data networks with a method known as encapsulation and tunneling: data packets are equipped with GPRS-specific protocol information and transferred between the MS and GGSN. In order to access the GPRS services, a MS first makes its presence known to the network by performing a GPRS attach. This operation establishes a logical link between the MS and the SGSN, and makes the MS available for, for example, paging via SGSN and notification of incoming GPRS data.
The SGSN is at the same hierarchical level as the MSC, keeps track of the individual MSs' location and performs security functions and access control. The Gateway GSN provides interworking with external packet-switched networks, and is connected with SGSNs via an IP-based GPRS backbone network.
FIG. 1 presents also exemplary protocol stacks that may be used in each network element for transmitting packet data. The GGSN 105 has protocol stack 115. The physical layer and the medium access layer are not specified and they are represented with symbols L1 and L2 in FIG. 1. The protocol on the medium access layer protocol is Internet Protocol (IP), and on IP both User Datagram Protocol (UDP) and Transfer Control Protocol (TCP) may be run. In GPRS core network, data is transmitted using GPRS Tunneling Protocol (GTP). Data that is carried in the GTP packets is either IP packets or X.25 packets, as specified by the upmost layer in the protocol stack 115.
Towards the GGSN the protocol stack 114 of the SGSN is similar to that of the GGSN. It lacks the upmost layer of the GGSN protocol stack because the data transmission protocol in GPRS core network is GTP. A base station controller and the base station connected to it form a base station system (BSS). The protocol stack 112 of a BSS is presented in FIG. 1, too. Towards a BSS the SGSN has a different protocol stack than towards to GGSN. The common physical layer of the SGSN and BSS is L1bis, and Frame relay is used in the second protocol layer. The upmost protocol layer between the SSGN and the BSS is Base Station System GPRS protocol (BSSGP). Over this protocol the SGSN still has Logical Link Control (LLC) and Sub-network Dependent Convergence Protocol (SNDCP). LLC and SNDCP connections are between the SGSN and a mobile station. The interface between a BSS and a SGSN is called Gb interface.
The base station system, or more precisely a base station, communicates with a mobile station using GSM RF as the physical layer. On this protocol there are Medium Access Control and Radio Link Control protocols. The base station system relays the data and signaling information between the RLC and BSSGP. The protocol stack 111 of a mobile station comprises LLC and SNDCP protocols on top of RLC protocol. On these protocols there is a packet data protocol which is common with the GGSN. The application is the upmost layer in the protocol stack.
The protocol stacks in FIG. 1 are those related to data transmission. Signaling, which relates, for example, to mobility management and resource reservation is carried out using GSM Mobility Management and Session Management (GMM/SM) protocol in the place of SNDCP. Otherwise the signaling protocol stacks are similar to the data transmission protocol stacks presented in FIG. 1.
In third generation future cellular networks, the base station subsystem comprises a controller, which in UMTS is called a radio network controller (RNC) and base stations connected to the RNC. The base stations are here referred to as third generation base stations (3G-BTS) in order to distinct them from the base stations of a GSM radio access network for example. FIG. 2 presents as an example of a third generation cellular network an UMTS radio access network. The mobile station 201 that is compatible with the UMTS network is different from a GSM mobile station 101. It communicates with a 3G-BTS 202 that is connected to a RNC 203. The RNC may be connected to a GPRS core network. This is in FIG. 2 marked by presenting the RNC connected to a GPRS supporting node 104.
FIG. 2 presents also the exemplary protocol stack 212 of the UMTS base station system. The protocol stack 212 is related to packet data. Towards a GPRS supporting node, the lowest protocol layer is the same as that in the protocol stack 112 of the GSM base station system, but the upper layers in these protocol stacks are different. In UMTS base station system, Asynchronous Transfer Mode (ATM) is used in the medium access layer and GPRS tunneling protocol is the upmost protocol.
Because the protocol stacks in the UMTS base station system and in a GPRS supporting node are different, there is need for an interworking unit. In FIG. 2, the interworking unit (IWU) 206 is presented as a separate device, but it may be a part of the RNC or the SGSN as well. Towards the UMTS radio access network the protocol stack 216 of the interworking unit is similar to that of the UMTS base station system, and towards the GPRS core network it is similar to the protocol stack which in an SGSN faces a radio access network. The protocol stack 216 has only three layers, and the upmost data transmission protocols are BSSGP and GTP. The interworking unit relays the BSSGP data packets further as GTP data packets and vice versa.
Signalling related to, for example, radio resource reservation and mobility management, is carried out using a Radio Access Network Application Part (RANAP). In signaling protocol stack, the RANAP replaces the GRPS tunneling protocol in the protocol stack 212 of the UTMS base station system and in the protocol stack 216 of the interworking unit.
FIG. 3 presents a schematic drawing of a network, where a GSM radio access network 300 and an UMTS radio access network 310 are connected to a GPRS core network 320. In FIG. 3, the GSM radio access network 300 comprises two base stations 102a and 102b, and a base station controller 103. The UMTS radio access network 310 comprises two 3G base stations 202a and 202b, and a radio access network controller 203. The GSM radio access network 300 is connected to the GPRS core network 320 by connecting the BSC 103 to a SGSN 104 of the GPRS core network 320. The UMTS radio access network 310 is connected to the GPRS core network 320 by connecting the RNC 203 to the same SGSN 104. The GPRS core network 320 is connected to a public data network 330 using a GGSN 105.
In the GPRS core network 320 between the SGSN and GGSN a data stream related to a certain connection is identified usually with a certain connection identifier, for example with a flow label. Each GTP packet carrying data related to, for example, a certain IP connection, has the same identifier.
In the GPRS core network, there are subscriber-specific or connection-specific queues for the data packets. For each subscriber there may be many GTP sessions, each of which has a unique identifier, for example the GTP flow label. In the GSM radio access network, the data packet queues are cell-specific, so that the management of the queues is easy in the BSC. Depending on the number of service classes, there may be many packet queues in a specific cell. In a SGSN, the BSSGP layer is responsible for re-organizing the subscriber-specific data packet queues to cell-specific queues. This re-organizing requires information on the subscriber identifier to which a certain GTP flow label relates and on the cell in which the subscriber is. The correspondence between a GTP flow label or other connection identifier and a subscriber identity may be determined, for example, in the process of radio access network resource reservations when a Packet Data Protocol (PDP) context is being set up.
In UMTS radio access network, the RNC expects the packets arriving from the GPRS core network to be organized in subscriber-specific queues. Therefore between the Gb and Iu interterfaces, for example in the IWU, the cell-specific data packet queues have to be re-organized to subscriber-specific queues. An example is presented in FIG. 4, which shows the BSSGP layer 400 and GTP layer 410 of an IWU 206. These layers are involved in transmission of user data, signaling data is transmitted using the BSSGP and the RANAP.
The cell-specific data packet queues 411-414 are shown in the BSSGP layer 400. In FIG. 4, the BSSGP layer comprises a switching entity 440, which is responsible for organizing the data packets to connection-specific queues 421-422. As an example, data packets 401-403 are shown to be heading to a certain cell in the UMTS radio access network 310. The data packets belong to different packet data connections, and therefore they are placed to separate transmission queues 421, 423 and 424. In FIG. 4, the switch management entity 441 comprises information about connections A, B, C and D. For these connections a PDP context has been established between a mobile station within the UPTS radio access network 310 and a GGSN. The information may be received, for example, from a subscriber database in a SGSN. In FIG. 4, a subscriber database 450 is presented and arrow 431 shows how the necessary information in the database is signaled to the switch management entity.
The problem is that in certain situations a SGSN may transmit packets towards a UMTS radio access network without checking if the receiver mobile station has successfully carried out resource reservation in the UMTS radio access network and has established a PDP context. In a handover from a GSM radio access network to an UMTS radio access network it may happen that the SGSN receives information from the GSM radio access network that a handover has been completed, but the UMTS radio access network has not yet reserved resources for the GPRS data related to this mobile station. The SGSN may direct downlink data at once to the UMTS radio access network, but in the IWU, or corresponding functionality incorporated to the RNC or SGSN, for example, there is no information about the PDP context. The IWU, for example, does not have a proper transmission queue where to place the data packets with a certain GTP flow label. It has to discard the data packets. Other packets heading to other mobile stations within the UMTS radio access network may suffer from additional delays due to the time consumed by the processing of the data packet without a proper PDP context. Further, if some data packets are deleted without informing the SGSN, it may send the packets again without realizing that the problem is actually the lack of reservations or an unestablished PDP context in the UMTS radio access network.